In the production of mechanical pulps, including TMP, RMP, and CTMP, refiners having relatively rotatable refiner elements are fed with cellulosic fibrous material that is to be refined into mechanical pulp. Typically, the positive chip or pulp flow through the refiner is dependent upon the refiner's own transporting capability. A typical refiner has a considerably high transporting capability due to high centrifugal forces that are generated. The capacity of the refiner system is generally determined by the transporting capability of the refiner, and control of the flow of pulp and steam out of the refiner. Conventionally, refiners are fed utilizing with one or more standard screw conveyors having generally cylindrical shafts and flights in constant diameter conduits, such as shown in Canadian patent No. 1079559.
According to the present invention it has been found that when a refiner is force fed--rather than merely relying upon the refiner's own transporting capabilities--pulp having given freeness, tensile and tear strength, and light scattering abilities can be produced with less energy. Alternatively, using the same amount of energy as when one relies upon the refiner's own transporting capabilities, by force feeding the refiner one can obtain a more desirable pulp, i.e. one having lower freeness, greater light scattering coefficient, greater tensile strength, and greater tear strength (over a wide variety of energy values).
Force feeding of a refiner is preferably accomplished according to the invention by utilizing a progressive compacting plugscrew. Such a screw is a standard piece of equipment in the pulp and paper industry for transporting pulp or chips from atmospheric presteaming into a preheating conveyor which operates at a pressure comparable to that of a refiner, and in other situations where it is desirable to develop a plug of chips which substantially prevents the flow of steam, or other gases, therethrough, including with refiners (e.g. see U.S. Pat. Nos. 4,457,804 and 3,327,952). A plugscrew comprises a shaft having conically tapered flights, rotatable in a passageway that is conically tapered in sympathy with the conical tapering of the flights, so that as the cellulosic fibrous material is transported by the rotating screw air is expelled therefrom and it is compacted.
According to the method of the present invention, cellulosic fibrous material is refined to produce mechanical pulp using a mechanical refiner having a given transporting capacity. The method comprises the step of (a) force feeding the refiner (e.g. with a progressive compacting plugscrew) with cellulosic fibrous material at a rate greater than the transporting capacity of the refiner. It is desirable to feed the refiner with a feed screw that has a transporting capacity about 10-40% greater than that of the refiner itself. Preferably there also is provided the step of regulating production of pulp by sensing the axial force on the rotor shaft of the refiner and controlling the spacing between the refiner elements in response to the sensing. Screw compaction is achieved by both conicity of the screw and progression in the screw. The compaction ratio should be at least 3/1 for wood chips and 6/1 for pulp. The screw speed of rotation should be at least 1/100 of the refiner rpm (e.g. about 6-10%).
The benefits achieved according to the invention are enhanced when the refiner that is utilized is a conical refiner, particularly a low frequency conical refiner such as shown in U.S. Pat. No. 4,754,935 and co-pending U.S. application Ser. No. 07/070,212, filed July 6, 1987 as a continuation of Ser. No.07/008,667 filed Jan. 30, 1987, now abandoned. Such a refiner has steam removal means within an actual grinding area between the refiner elements, and a centrifugal separator associated with the rotor shaft for centrifugally separating steam and fibers, and allows for effective, low energy production of mechanical pulp. When the force feeding according to the present invention is properly practiced so that a plug of chips (cellulosic fibrous material) forms that prevents passage of steam out of the refiner inlet, the pulp produced will have a lower freeness, greater light scattering coefficient, greater tensile strength, and--over a wide variety of energy levels--greater tear strength, than pulp produced without force feeding of the refiner, for a given amount of energy.
According to another aspect of the present invention, a mechanical cellulosic fibrous material pulp having good freeness, light scattering, tensile strength, and tear properties is produced for a given energy input, utilizing a mechanical refiner. The pulp is produced by the step of force feeding the refiner with cellulosic fibrous material at a rate about 1O-40% greater than the transporting capacity of the refiner.
According to another aspect of the present invention, there is provided an apparatus for producing pulp from cellulosic fibrous material. The apparatus comprises: (a) A mechanical refiner having at least two relatively movable refining elements, and a rotor shaft connected to one of the elements, a material inlet, and a pulp outlet, and a given transporting capacity. And, (b), means for force feeding the refiner inlet with material at a rate greater than the transporting capacity of the refiner. The means (b) preferably comprises a progressive compacting plugscrew. The refiner (a) preferably is a conical refiner with means for adjusting the spacing between the refining elements, steam removal means, and a centrifugal separator--e.g. a low frequency refiner. Means are provided for sensing the axial force on the rotor shaft and in response to the sensed force actuating the means for adjusting the spacing between the conical refining elements to control production.
It is the primary object of the present invention to provide for the production of mechanical pulp that has enhanced properties, at a given energy input level, by force feeding a refiner. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.